Does document.body.innerHTML = "" clear the web page?

Whilst agreeing with Douwe Maan and Erik's answers, there are a couple of other things here that you may find useful.

Firstly, within your head tags, you can reference a separate JavaScript file, which is then reusable:

<script language="JavaScript" src="/common/common.js"></script>

where common.js is your reusable function file in a top-level directory called common.

Secondly, you can delay the operation of a script using setTimeout, e.g.:

setTimeout(someFunction, 5000);

The second argument is in milliseconds. I mention this, because you appear to be trying to delay something in your original code snippet.

To expound on Douwem's answer, in your script tag, put your code in an onload:

<script type="text/javascript">
    window.onload=function(){
       document.body.innerHTML = "";
    }
</script>

document.body.innerHTML = ''; does clear the body, yeah. But it clears the innerHTML as it is at the moment the code is ran. As you run the code before the images and the script are actually in the body, it tries to clear the body, but there's nothing to clear.

If you want to clear the body, you have to run the code after the body has been filled with content. You can do this by either placing the <script> block as the last child of body, so everything is loaded before the code is ran, or you have to use some way to listen to the dom:loaded event.